# Patent

## Title
Low viscosity/high conductivity sodium haloaluminate electrolyte

## Abstract

An additive that is added to the NaAlX4 electrolyte for use in a ZEBRA battery (or other similar battery). This additive has a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties, thereby freeing some Na+ ions to transport (move). By using a suitable NaAlX4 electrolyte additive, the battery may be operated at much lower temperatures than are typical of ZEBRA batteries (such as, for example, at temperatures between 150 and 200° C.). Additionally, the additive also lowers the viscosity of the electrolyte solution and improves sodium conductivity. Non-limiting examples of the additive SOCl2, SO2, dimethyl sulfoxide (DMSO, CH3SOCH3), CH3S(O)Cl, SO2Cl2. A further advantage of using this additive is that it allows the use of a NaSICON membrane in a ZEBRA-type battery at lower temperatures compared to a typical ZEBRA battery.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent Application No. 61/831,691 filed Jun. 6, 2013, entitled LOW VISCOSITY/HIGH CONDUCTIVITY SODIUM HALOALUMINATE ELECTROLYTE. The foregoing application is incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to batteries. More specifically, the present embodiments relate to methods for lowering the viscosity and increasing the conductivity of a sodium haloaluminate electrolyte used in an intermediate temperature sodium anode battery, thereby making the battery more efficient.
BACKGROUND OF THE INVENTION
A known type of battery is the so-called ZEBRA battery. Additional information regarding the “ZEBRA” battery (and a similar sodium/sulfur battery) can be found in the published literature, including Karina B. Hueso et al., “High temperature sodium batteries: status, challenges and future trends,” Energy Environ. Sci., 14 Jan. 2013 and J. L. Sudworth, “The Sodium/nickel chloride (ZEBRA) battery,” J. Power Sources, 100 (2001) pp. 149-163.
FIG. 1 shows a drawing of an exemplary cathode and anode that is used in a “ZEBRA” battery 100. The battery 100 includes a Ni cathode 116 and a molten Na metal anode 114. Electrolyte is used to carry charge between the anode and the cathode. This electrolyte may be a sodium haloaluminate (NaAlX4) material such as NaAlCl4, NaAlBr4, or NaAlI4. A NaSICON or Beta alumina membrane 115 is used to separate the anode from the electrolyte. The charge-discharge reactions are summarized below, for the embodiment where the halogen is chlorine:

(Anode) 2Na2Na++2e−

(Cathode) NiCl2+2Na++2e−Ni+2NaCl

(Overall cell) NiCl2+2NaNi+2NaCl, E=2.58V

The foregoing oxidation/reduction reactions of Na and Ni produce the charge within the battery 100. The cathode is fabricated of a porous structure of nickel. The pores are impregnated with an electrolyte comprising NaAlCl4. NaAlCl4 has a melting point of about 157° C. Thus, the ZEBRA battery must be operated at a sufficiently high temperature to ensure that the NaAlCl4 (or other sodium haloaluminate) is molten with a sufficiently low viscosity to enable penetration of the porous nickel cathode and sufficiently high conductivity of sodium ions.
In this battery during discharge, Na is oxidized at the anode to form sodium ions that transport across membrane 115. Nickel halide is converted to metallic nickel and sodium halide which becomes part of the molten NaAlX4 electrolyte. During charge, sodium ions are transported across membrane 115 and reduced at the anode. The Ni cathode is oxidized to NiX2.
One of the features of the ZEBRA battery is that it is typically operated at 300° C. or higher due to the ohmic resistance of the membrane 115 and the need to have the NaAlX4 be molten. The high temperatures are used to ensure that the NaAlX4 is molten and has a low viscosity so that the Na+ ions may transport between the cathode materials and the membrane 115.
At about 300° C., the bond between the Na ion and the [AlX4]− moiety weakens, as shown below:

Na+ - - - [AlX4]−

The dashed line indicates that the bond weakens such that, for some of the NaAlX4 species, the bond will actually break, thereby allowing Na+ ions to transport. It is this weakening/breaking of the bonds that allows the battery to operate.

However, operating the ZEBRA battery at 300° C. can be cost-prohibitive as it requires a tremendous amount of energy to maintain the battery at that high temperature. Accordingly, there is a need in the art for a new type of battery that is similar to the ZEBRA battery that may be operated at lower temperatures. Such a device is disclosed herein.
BRIEF SUMMARY OF THE INVENTION
The present embodiments relate to an additive that is added to sodium haloaluminate (NaAlX4) electrolyte for use in a ZEBRA battery (or other similar battery). The NaAlX4 electrolyte may be represented by the formula Na+[AlX4]−, where X is a halogen and an ionic bond exists between the Na+ and [AlX4]− moieties. The additive interacts with the [AlX4]− moiety of the NaAlX4, thereby weakening the ionic bond with the Na+ ion and allowing some of the sodium ions to transport (move). In some non-limiting embodiments, the additive increases sodium conductivity by at least 10%. In other non-limiting embodiments, the additive increases sodium conductivity by at least 20%.
By using this additive, the battery may be operated at much lower temperatures than are typical of ZEBRA batteries. In some embodiments, this additive may allow the battery to be operated at temperatures in the range from 150 to 200° C. Additionally, the additive also lowers the viscosity of the electrolyte. In some non-limiting embodiments, the viscosity of the electrolyte is lowered by about 50%.
The additive may be combined with the sodium haloaluminate in an amount less than 50 mole % of the sodium haloaluminate.
The additive preferably has a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties. In some non-limiting embodiments, the additive is selected from the following group of compounds: SOCl2, SO2, dimethyl sulfoxide (DMSO, CH3SOCH3), CH3S(O)Cl, and SO2Cl2.
A further advantage of using this additive is that it may allow the use of a NaSICON membrane in the ZEBRA battery rather than a Beta Alumina membrane. NaSICON membranes provide specific advantages over Beta Alumina membranes, and thus it is beneficial to be able to use this type of membrane.
In some non-limiting embodiments, the halogen X is selected from chlorine, bromine, and iodine.
The disclosed invention also includes a molten sodium battery containing an electrolyte and additive. The battery includes a molten sodium metal negative electrode, which electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to sodium metal during recharging. The battery further includes a positive electrode compartment comprising a positive electrode disposed in a positive electrolyte. The positive electrolyte may include a quantity of sodium haloaluminate as disclosed herein, represented by the formula Na+[AlX4]−, where X is a halogen, wherein an ionic bond exists between the Na+ and [AlX4]− moieties. The sodium haloaluminate has a viscosity and a sodium ion conductivity. The positive electrolyte includes a quantity of an additive, as described herein, that lowers the viscosity of the sodium haloaluminate and increases the sodium ion conductivity of the sodium haloaluminate.
The molten sodium battery contains a sodium ion conductive electrolyte membrane that separates the molten sodium metal negative electrode from the positive electrolyte. In some non-limiting embodiment, the sodium ion conductive electrolyte membrane comprises a NaSICON-type material. In some non-limiting embodiments, the NaSICON-type material comprises a composite membrane having a porous layer and a dense functional layer. In operation, the sodium metal negative electrode is in contact with the conductive electrolyte membrane as the battery operates. The battery functions at an operating temperature between about 150° C. and about 200° C.
The disclosed invention includes a method of lowering the viscosity and increasing the sodium ion conductivity of a sodium haloaluminate battery electrolyte. The method includes obtaining a quantity of sodium haloaluminate, wherein the sodium haloaluminate is represented by the formula Na+[AlX4]−, wherein an ionic bond exists between the Na+ and [AlX4]− moieties; and adding a quantity of an additive, as described herein, to the sodium haloaluminate to lower the viscosity of the sodium haloaluminate and increase the sodium ion conductivity of the sodium haloaluminate.



BRIEF DESCRIPTION OF THE DRAWINGS
In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
FIG. 1 shows an example of a battery, and more specifically, a ZEBRA battery;
FIG. 2 shows the chemical interactions between an additive (SOCl2) and the NaAlX4 species, as described herein.
FIG. 3 shows the chemical interactions between an additive (SO2) and the NaAlX4 species, as described herein.
FIG. 4 shows the chemical interactions between an additive (DMSO, CH3SOCH3) and the NaAlX4 species, as described herein.
FIG. 5 shows the chemical interactions between an additive (CH3S(O)Cl) and the NaAlX4 species, as described herein.
FIG. 6 shows the chemical interactions between an additive (SO2Cl2) and the NaAlX4 species, as described herein.



DETAILED DESCRIPTION OF THE INVENTION
The present embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and batteries of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of present embodiments of the invention.
The present embodiments relate to additives that may be added to a sodium haloaluminate (NaAlX4) electrolyte as a means of lowering the viscosity of this material, and thereby lowing the temperature required to operate the battery.
Specifically, as shown in FIG. 2, the electrolyte will include NaAlX4. (In the embodiments shown in FIG. 2, the halide (X−) moiety is a chloride (Cl−) moiety; however, a bromide (Br−) moiety or an iodide (I−) moiety may likewise be used as the halide (X−) moiety. The electrolyte also includes an additive, which in the embodiment of FIG. 2 comprises thionyl chloride (SOCl2). Because the O and Cl moieties of the thionyl chloride are more electronegative than the sulfur, these moieties withdraw electrons from the sulfur (e.g., are “electron withdrawing groups”). This causes a slight positive charge on the sulfur atom (as represented by the “δ+” superscript). This slight positive charge of the sulfur is attracted to the negatively charged X− moiety of the [AlX4]− ion. In turn, this attraction between the S and the halide moiety of the [AlX4]− ion weakens the bond between the sodium ion and the [AlX4]− ion, thereby allowing some of the Na+ ions to move throughout the electrolyte. At the same time, the additive SOCl2 also operates to decrease the viscosity of the electrolyte. This reduction in viscosity further allows the Na+ ions to flow through the system.
FIG. 3 shows the chemical interactions between an additive, sulfur dioxide (SO2) and the NaAlX4 species, in a manner similar to thionyl chloride described above. The sulfur atom has a slight positive charge on the sulfur atom (as represented by the “δ+” superscript). This slight positive charge of the sulfur is attracted to the negatively charged X− moiety of the [AlX4]− ion. In turn, this attraction between the S and the halide moiety of the [AlX4]− ion weakens the bond between the sodium ion and the [AlX4]− ion, thereby allowing some of the Na+ ions to move throughout the electrolyte. At the same time, the additive SO2 also operates to decrease the viscosity of the electrolyte. This reduction in viscosity further allows the Na+ ions to flow through the system.
FIG. 4 shows the chemical interactions between an additive, dimethyl sulfoxide (DMSO, CH3SOCH3) and the NaAlX4 species, in a manner similar to thionyl chloride described above. The sulfur atom has a slight positive charge on the sulfur atom (as represented by the “δ+” superscript). This slight positive charge of the sulfur is attracted to the negatively charged X− moiety of the [AlX4]− ion. In turn, this attraction between the S and the halide moiety of the [AlX4]− ion weakens the bond between the sodium ion and the [AlX4]− ion, thereby allowing some of the Na+ ions to move throughout the electrolyte. At the same time, the additive DMSO also operates to decrease the viscosity of the electrolyte. This reduction in viscosity further allows the Na+ ions to flow through the system.
FIG. 5 shows the chemical interactions between an additive, CH3S(O)Cl and the NaAlX4 species, in a manner similar to thionyl chloride described above. The sulfur atom has a slight positive charge on the sulfur atom (as represented by the “δ+” superscript). This slight positive charge of the sulfur is attracted to the negatively charged X− moiety of the [AlX4]− ion. In turn, this attraction between the S and the halide moiety of the [AlX4]− ion weakens the bond between the sodium ion and the [AlX4]− ion, thereby allowing some of the Na+ ions to move throughout the electrolyte. At the same time, the additive CH3S(O)Cl also operates to decrease the viscosity of the electrolyte. This reduction in viscosity further allows the Na+ ions to flow through the system.
FIG. 6 shows the chemical interactions between an additive, sulfuryl chloride (SO2Cl2) and the NaAlX4 species, in a manner similar to thionyl chloride described above. The sulfur atom has a slight positive charge on the sulfur atom (as represented by the “δ+” superscript). This slight positive charge of the sulfur is attracted to the negatively charged X− moiety of the [AlX4]− ion. In turn, this attraction between the S and the halide moiety of the [AlX4]− ion weakens the bond between the sodium ion and the [AlX4]− ion, thereby allowing some of the Na+ ions to move throughout the electrolyte. At the same time, the additive sulfuryl chloride (SO2Cl2) also operates to decrease the viscosity of the electrolyte. This reduction in viscosity further allows the Na+ ions to flow through the system.
By having the Na+ ions transport through the system, the ZEBRA battery can be used to store power and subsequently release power, as desired. At the same time, the fact that the Na+ ions can transport through the system means that the battery can be operated at lower temperatures (of between 150 to 200° C.) rather than the traditional temperature of 300° C. or greater. This lower operating temperature is much easier to achieve and maintain. Furthermore, a lower operating temperature results in costs savings as less resources need to be devoted to heating the battery to the proper temperature.
Further, because lower temperatures are available, the sodium ion conductive electrolyte membrane is no longer required to be Beta Alumina. Rather, embodiments may be constructed in which the membrane is made of NaSICON. NaSICON is a membrane material that is commercially available from Ceramatec, Inc. of Salt Lake City, Utah. U.S. Patent Application Publication No. 20070138020 describes the structure and properties of NaSICON as well as other membrane materials that may be used in the present embodiments. The entire disclosure of this published U.S. application is expressly incorporated herein by reference.
One of the features of NaSICON is the ability to create two distinctive environments on different sides of the membrane. This means that the solutions for the anolyte and catholyte may be different, the pressures on each side of the membrane may be different, the reactants and reaction conditions on each side of the membrane may be different, etc. In other words, the designer of the battery or secondary cell can tailor/select reactants/conditions for both the anolyte and catholyte that optimize each specific reaction. In some embodiments, the NaSICON membrane may have excellent conductivity (such as up to 100 mS/cm at 175° C.). The NaSICON membrane can be a supported membrane that is between 50-25 microns thick.
Additional embodiments may be designed in which the additive is a different chemical other than thionyl chloride, SOCl2, SO2, dimethyl sulfoxide (DMSO, CH3SOCH3), CH3S(O)Cl, SO2Cl2, described above. It is understood that corresponding chemicals using a different halogen other than chlorine may be used. Moreover, other suitable additive chemicals may be used that have a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties. Thus, other chemicals (such as polar chemicals) that can interact with the [AlX4]− moieties in the manner outlined above and allow the Na+ ions to transport may be used as the electrolyte additive. These electrolyte additives may operate to lower the viscosity of the electrolyte.
It should be noted that with respect to SO2, this chemical may be used as either a gas or a liquid. Since the NaSICON membrane allows for different reaction conditions on either side of the membrane, the side with the SO2 could be pressurized so that the SO2 is in the liquid form.
All the patent applications and patents listed herein are expressly incorporated herein by reference.

## Claims

1. A molten sodium battery cathode electrolyte consisting of:
a quantity of molten sodium haloaluminate having a viscosity and a sodium ion conductivity, wherein the sodium haloaluminate is represented by the formula Na+[AlX4]−, where X is a halogen, wherein an ionic bond exists between the Na+ and [AlX4]− moieties; and
a quantity of an additive that lowers the viscosity of the sodium haloaluminate and increases the sodium ion conductivity of the sodium haloaluminate at a temperature in the range from 150 to 200° C., wherein the additive is present in an amount less than 50 mole % of the sodium haloaluminate.

2. The molten sodium battery cathode electrolyte of claim 1, wherein the additive has a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties.

3. The molten sodium battery cathode electrolyte of claim 1, wherein the additive is selected from the group consisting of SOCl2, SO2, DMSO (CH3SOCH3), CH3S(O)Cl, and SO2Cl2.

4. The molten sodium battery cathode electrolyte of claim 1, wherein X is selected from chlorine, bromine, and iodine.

5. The molten sodium battery cathode electrolyte of claim 1, wherein the viscosity of the electrolyte is lowered by about 50%.

6. The molten sodium battery cathode electrolyte of claim 1, wherein the sodium conductivity is increased by at least 10%.

7. A molten sodium battery comprising:
a molten sodium metal negative electrode, which electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to sodium metal during recharging;
a positive electrode compartment comprising a positive electrode disposed in a positive electrolyte, wherein the positive electrolyte consists of:
a quantity of molten sodium haloaluminate having a viscosity and a sodium ion conductivity, wherein the sodium haloaluminate is represented by the formula Na+[AlX4]−, where X is a halogen, wherein an ionic bond exists between the Na+ and [AlX4]− moieties; and
a quantity of an additive that lowers the viscosity of the sodium haloaluminate and increases the sodium ion conductivity of the sodium haloaluminate, wherein the additive is present in an amount less than 50 mole % of the sodium haloaluminate; and

a sodium ion conductive electrolyte membrane that separates the molten sodium metal negative electrode from the positive electrolyte,
wherein the sodium metal negative electrode is in contact with the conductive electrolyte membrane as the battery operates, and wherein the battery functions at an operating temperature between about 150° C. and about 200° C.

8. The molten sodium battery of claim 7, wherein the positive electrolyte additive has a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties.

9. The molten sodium battery of claim 7, wherein the positive electrolyte additive is selected from the group consisting of SOCl2, SO2, DMSO (CH3SOCH3), CH3S(O)Cl, and SO2Cl2.

10. The molten sodium battery of claim 7, wherein the sodium ion conductive electrolyte membrane comprises a NaSICON-type material.

11. The molten sodium battery of claim 10, wherein the NaSICON-type material comprises a composite membrane having a porous layer and a dense functional layer.

12. The molten sodium battery of claim 7, wherein X is selected from chlorine, bromine, and iodine.

13. A method of lowering the viscosity and increasing the sodium ion conductivity of a molten sodium battery, molten sodium haloaluminate cathode electrolyte comprising:
obtaining a quantity of sodium haloaluminate cathode electrolyte consisting of sodium haloaluminate, wherein the sodium haloaluminate is represented by the formula Na+[AlX4]−, wherein an ionic bond exists between the Na+ and [AlX4]− moieties;
adding a quantity of an additive to the sodium haloaluminate to lower the viscosity of the sodium haloaluminate and increase the sodium ion conductivity of the sodium haloaluminate, wherein the additive is added in an amount that is less than 50 mole % of the sodium haloaluminate; and
heating the sodium haloaluminate to temperature in the range from 150 to 200° C.

14. The method of claim 13, wherein the additive has a moiety with a partial positive charge (δ+) that attracts the negative charge of the [AlX4]− moiety and weakens the ionic bond between the Na+ and [AlX4]− moieties.

15. The method of claim 13, wherein the additive is selected from the group consisting of SOCl2, SO2, DMSO (CH3SOCH3), CH3S(O)Cl, and SO2Cl2.

16. The method of claim 13, wherein X is selected from chlorine, bromine, and iodine.

17. The method of claim 13, wherein the viscosity of the electrolyte is lowered by about 50%.

18. The method of claim 13, wherein the sodium conductivity is increased by at least 10%.